1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method or an electrostatic recording method. In particular, the present invention relates to an image forming apparatus including an intermediate transfer belt and a transfer conveyance belt.
2. Description of the Related Art
Conventionally, there is a color image forming apparatus which can form a full-color image using a direct transfer method or an intermediate transfer method.
The intermediate transfer method enables the image forming apparatus to easily form an image on various types of recording materials, so that selectivity of recording materials can be improved. In the intermediate transfer method, a toner image formed on a single photosensitive drum or a plurality of photosensitive drums is once transferred (i.e., primary transferred) to the intermediate transfer belt, i.e., a rotatable belt member. The toner image on the intermediate transfer belt is then transferred (secondary transferred) to the recording material.
FIG. 7 illustrates a configuration of an image forming apparatus using the intermediate transfer method. Referring to FIG. 7, the image forming apparatus includes four process units, i.e., image forming units, corresponding to four colors, i.e., yellow, magenta, cyan, and black, respectively. The image forming apparatus further includes the following: photosensitive drums (image bearing member) 10a, 10b, 10c, and 10d, charging devices 20a, 20b, 20c, and 20d, exposure units 30a, 30b, 30c, and 30d, developing devices 40a, 40b, 40c, and 40d, an intermediate transfer belt 510, primary transfer members 530a, 530b, 530c, and 530d, photosensitive drum cleaners 60a, 60b, 60c, and 60d, secondary transfer members 560 and 570, an intermediate transfer belt drive roller 520, and a fixing device 70.
In an image forming process, the charging devices 20a, 20b, 20c, and 20d uniformly charge the photosensitive drums 10a, 10b, 10c, and 10d, respectively. The exposure units 30a, 30b, 30c, and 30d then expose the photosensitive drums 10a, 10b, 10c, and 10d with a light beam modulated according to an image signal, so that electrostatic latent images are formed on the photosensitive drums 10a, 10b, 10c, and 10d. 
The developing devices 40a, 40b, 40c, and 40d develop the electrostatic latent images to form toner images on the photosensitive drums 10a, 10b, 10c, and 10d. The toner images are then sequentially transferred to the intermediate transfer belt 510 by applying a transfer bias on the primary transfer members 530a, 530b, 530c, and 530d. The toner remaining on the photosensitive drums 10a, 10b, 10c, and 10d after the image transfer is collected by the photosensitive drum cleaners 60a, 60b, 60c, and 60d. 
The images that are sequentially superposed on the intermediate transfer belt 510 from the photosensitive drums 10a, 10b, 10c, and 10d are then transferred to a recording material P by applying a secondary transfer bias between the secondary transfer members 560 and 570. The fixing device 70 fixes the toner image transferred to the recording material P, so that a full color image can be acquired.
Among image forming apparatuses using the intermediate transfer belt as described above, there is an apparatus which includes a transfer conveyance belt for conveying the recording material (refer to Japanese Patent Application Laid-Open No. 2008-14989). By using both the intermediate transfer belt and the transfer conveyance belt, recording materials such as thin paper can be stably conveyed, so that a stable image forming process can be performed on recording materials of greater diversity.
FIG. 8 illustrates an image forming apparatus employing an intermediate transfer belt and a transfer conveyance belt. Referring to FIG. 8, the image forming apparatus includes four process units, i.e., image forming units, corresponding to four colors, i.e., yellow, magenta, cyan, and black. The image forming apparatus further includes the following: photosensitive drums (image bearing member) 10a, 10b, 10c, and 10d, charging devices 20a, 20b, 20c, and 20d, exposure units 30a, 30b, 30c, and 30d, developing devices 40a, 40b, 40c, and 40d, an intermediate transfer belt 510, an intermediate transfer belt drive roller 520, primary transfer members 530a, 530b, 530c, and 530d, photosensitive drum cleaners 60a, 60b, 60c, and 60d, a transfer conveyance belt 910, a transfer conveyance belt drive roller 920, secondary transfer members 560 and 570, an intermediate transfer belt drive roller 520, and a fixing device 70.
In the image forming process, an image is formed on the intermediate transfer belt 510, similarly as in the above-described image forming apparatus using the intermediate transfer method. The recording material P is then mounted on the transfer conveyance belt 910 and conveyed to a secondary transfer nip N2. By applying a secondary transfer bias between the secondary transfer members 560 and 570, the toner image on the intermediate transfer belt 510 is transferred to the recording material P at the secondary transfer nip N2. The recording material P to which the image is transferred is separated from the transfer conveyance belt 910 in the vicinity of the transfer conveyance belt drive roller 920. The fixing device 70 then fixes the toner image on the recording material P, so that a full color image can be acquired.
However, in the above-described conventional image forming apparatus using the intermediate transfer belt and the transfer conveyance belt, problems may occur as described below, depending on the arrangement of the drive rollers 520 and 920.
A surface of the intermediate transfer belt 510 suspended between the inner secondary transfer roller 560 and the intermediate transfer belt drive roller 520 will be referred to as Mf. Further, a surface of the transfer conveyance belt 910 suspended between the outer secondary transfer roller 570 and the transfer conveyance belt drive roller 920 will be referred to as Mh.
Moreover, the rotational speeds of the drive rollers 520 and 920 are set so that the moving speed of the intermediate transfer belt 510 becomes greater than the moving speed of the transfer conveyance belt 910. In such a case, the transfer conveyance belt 910 applies a force on the intermediate transfer belt 510 in a direction of reducing the rotational speed of the intermediate transfer belt 510 at the secondary transfer nip N2. A pulling force is thus applied on the surface Mf of the intermediate transfer belt 510. As a result, the intermediate transfer belt 510 is firmly wound around the intermediate transfer belt drive roller 520, so that the intermediate transfer belt 510 does not slip.
However, at the same time, the intermediate transfer belt 510 applies a force on the transfer conveyance belt 910 in a direction of increasing the rotational speed of the transfer conveyance belt 910 at the secondary transfer nip N2. As a result, the surface Mh of the transfer conveyance belt 910 becomes slackened. Therefore, the transfer conveyance belt 910 becomes less firmly wound around the transfer conveyance belt drive roller 920, so that the transfer conveyance belt 910 may slip and cause problems such as image expansion and contraction.
On the other hand, if the moving speed of the transfer conveyance belt 910 is set to be greater than the moving speed of the intermediate transfer belt 510, the winding of the intermediate transfer belt 510 onto the intermediate transfer belt drive roller 520 becomes unstable. As a result, color misregistration and image expansion and contraction may be generated.
As described above, since there are manufacturing variations in the radii of the drive rollers, it is difficult for the moving speed of the intermediate transfer belt 510 to be the same as the moving speed of the transfer conveyance belt 910. Further, the rotations of the drive rollers are uneven, so that the moving speed of the intermediate transfer belt 510 becomes unstable, being greater or less than the moving speed of the transfer conveyance belt 910. Such instability is generated even if the rotational speeds of the drive rollers are set so that the moving speeds of the intermediate transfer belt 510 and the transfer conveyance belt 910 become the same. As a result, the intermediate transfer belt 510 becomes more easily affected by a backlash in a driving force transmission portion such as a gear, increasing variations in the images.
Japanese Patent Application Laid-Open No. 2001-337538 discusses placing the drive roller of the intermediate transfer belt upstream of the secondary transfer portion N2 with respect to the moving direction of the intermediate transfer belt. Further, it is desirable to place the drive roller of the transfer conveyance belt downstream of the secondary transfer portion N2.
However, the problem may occur when the moving speed of the image bearing member is greater than the moving speed of the intermediate transfer belt even if the drive roller is disposed as described above. As a result, the rotation of the intermediate transfer belt becomes unstable and thus affects the rotation of the transfer conveyance belt.
Japanese Patent Application Laid-Open No. 10-186786 discusses a relationship among the rotational speed of the image bearing member, the rotational speed of the intermediate transfer belt, and the moving speed of the recording material. However, the positions of the intermediate transfer belt and the drive roller are not discussed, and the rotation becomes unstable if the transfer conveyance belt is further disposed in such configuration.